The medical field is rapidly advancing towards miniaturizing active implanted medical devices (AIMD), allowing for smaller devices to be placed closer to targeted organs. This trend enables less invasive and more cost-effective implantation procedures, opening new possibilities in neuromodulation, cardiac sensing, and smart orthopaedics. However, product designers face challenges due to the limited space for the energy pack powering these devices. The energy pack typically includes a battery, capacitance for power regulation, a wireless charge transfer receiver, and a power management system to ensure safe, efficient coordination of all components. This article explores how Cirtec Medical’s application-specific integrated circuit (ASIC) design expertise, combined with Ilika’s solid-state battery (SSB) technology, enables high-performance, miniaturized AIMD systems with efficient power architecture.

Most AIMD operate somewhere on the spectrum between sensing and stimulation. Due to the need to be minimally invasive and biomedically inert, obtrusive energy storage components and accompanying controllers must either judiciously conserve energy or remain in continuous contact with a larger energy reserve. To conserve energy, a duty cycle is employed where the system activates, performs the primary function (sensing, stimulation, or computation), and then sleeps. Where necessary, additional energy may be required to receive/transmit conditioned data from/to an external reader. The duty cycle can vary, from pulses multiple times per second to hours or days apart. Alternatively, a continuous energy reserve relaying power to an implant either requires a perpetually open incision to pass power through a wire or wirelessly transmitting energy, which is limited by specific absorption rate (SAR) as well as resultant radiated noise on the power plane.

While biomedically compliant form-factor AIMDs have been previously developed to use non-rechargeable (primary) batteries, the functionality of the AIMD becomes either severely energy-constrained (i.e., reduced functionality) or a very low duty cycle. In both cases, when the battery is exhausted, the patient must go through risky explant procedures. In contrast, brief scheduled wireless power transfer to rechargeable batteries can provide a connection to a large energy reserve within safe energy transmission limits and then allow the AIMD to operate in a low-noise state while running from the battery. Additionally, with rapid recharging (under 10 minutes), patient compliance with clinical treatment scheduling can be improved. While rechargeable batteries are not yet approved for life-sustaining applications, 15% of non-life-critical implanted devices use rechargeable lithium-ion batteries.

SSB are ideally situated to address these constraints and expand into new markets. They are millimetre scaled and yet have a high energy density, long lifespans, and can implement rapid charging. The Stereax M300, for example, is an SSB designed by Ilika Technologies specifically to power AIMD. It has a capacity of 300 µAh and can deliver 3 mA of pulsed current, all within a form factor of just 5.6 mm x 3.6 mm x 1.1 mm and a mass of 40 mg. These batteries can be stacked to increase voltage or capacity, making them adaptable for various applications. Further, the Stereax M300 can be readily integrated into standard ASIC processing, allowing for rapid development, testing, and end-to-end quality control. Ilika is partnering with Cirtec Medical to enable visionary product designers in medtech to implement this full system-on-chip (SoC) functionality, from chip-based transceiver coils and wireless power conditioning to industry-leading AIMD design and encapsulation. Cirtec’s vertically integrated capabilities, ranging from ASIC development and in-house test systems to full-scale AIMD assembly and sterilization, allow for seamless integration of solid-state batteries into SoC platforms.

Opportunities in Wireless Power Regulation

The Stereax SSB is ideally suited to provide this energy reservoir with a wirelessly powered ASIC. A persistent problem in this space involves dealing with the problem of intermittent power. When a SoC starts up, there is an inrush of current required to “wake” the power conditioning circuitry. This start-up state needs to be precisely controlled and can involve rapid changes in the load impedance as analog blocks come on-line. This, in turn, complicates the matching between the on-chip receiving coil and the external reader. Poor management of this interplay can lead to transmitted power exceeding SAR limits, depending on the transmitter-to-receiver transmission efficiency, and retard the SoC from reaching stable operation. With the consistent energy reserve and dampening tank capacitor banks, a 2-stage approach with a cold-start facility followed by a boost stage enables a Maximum Power Point Tracking algorithm to optimize harvesting efficiency via load balancing. An additional benefit of this rapid wake-up to stable operation is being able to lower the duty cycle for the heavy power draw accompanying the primary ASIC functionality.

Opportunities in Wireless Power Memory Maintenance

In those power-saving periods where the primary ASIC functionality sleeps, an SSB again provides invaluable assistance. Without a continuous power supply, the factory default digital start-up state of an ASIC must be loaded from one-time programmable memory. Alternatively, the SoC could employ Flash Memory, but this method requires high voltage programming of 5V or greater, which stands in contrast to the low voltage operation of many modern AIMD, less than 1.8V. To provide more flexibility, speed, and consistent states between intermittent power delivery periods, volatile static random-access memory (SRAM) should be implemented. Due to Stereax SSB’s low quiescent current of a few nA, a SoC can maintain SRAM memory across long periods of received power droughts.

Ensuring Safe, Long Lifetime AIMD Operation via ASIC and SSB Integration

As rechargeable batteries are increasingly used in AIMD, ensuring their efficient and safe charging, storage, and discharge is crucial for their operational lifetime. ASIC power conditioning designed by Cirtec provides regulated voltage outputs for both AIMD operation as well as safe charging management for the battery. SSB is a variant of lithium-ion batteries and, like them, requires its voltage to be kept within a safe window during operation, and the charge and discharge currents to be limited to values that will not create irreversible damage to the battery. Pulsed loads complicate this, as current pulses can temporarily dip the voltage: to prevent disconnection due to these dips, a hysteresis would be introduced to allow the voltage to recover before disconnecting, ensuring the full battery capacity is used. To do this, the ASIC would isolate the battery when needed, provide capacitive support, and be programmable for adjustable operational parameters. Cirtec ASICs are custom-designed to include programmable cold start logic, battery disconnect protection, and multi-rail voltage regulation, all critical to safe, long-duration implant operation. These circuits are tested for reliability across thermal and bio-environmental stress scenarios typical in implantable systems.

Summary

ASICs are the usual solutions providing the features necessary for the efficient, long-term operation of battery-powered designs. The correct power management solution is dependent on the individual requirements of each application and the specifications of the battery. When using a microprocessor-based board, the microprocessor itself can be utilised to offer additional information on the condition of the battery and provide a further degree of control over the power management solution. Using this Application Note to select the ideal power management solution for your application should ensure that the battery operates to its full potential for its expected lifetime. Ilika’s partnership with Cirtec leverages Cirtec’s full stack development capability, from ASIC and firmware co-design to implant assembly and regulatory strategy, providing device developers a single source solution to deliver miniaturized, rechargeable implants to market faster and with greater confidence

Denis Pasero joined Ilika Technologies in 2008, as a scientist specializing in battery technology, to manage commercial lithium ion projects. He became part of the Ilika team to apply his strong academic knowledge to commercial applications and saw the potential to be part of the development and success story of an enterprising smaller company with exciting technology and novel product ideas. Today, as Product Commercialization Manager, Denis interfaces between customers and technical teams.

Gregory E. Moore received his Ph.D. in electrical engineering from the University of Washington, Seattle, WA, USA, in 2023 with B.S. degrees in electrical engineering and physics from the University of Maryland, College Park, MD, USA. Prior to returning to academia, he worked as an Electrical Engineer with Tao of Systems Integration, Inc., Hampton, VA, USA, where he was engaged in mixed-signal circuit and control systems’ design for flow measurement products for aeronautics/marine engineering. While pursuing his PhD as a Research Assistant with the University of Washington, his research centered around WPT and low-power communication design, with a special focus on applications directed toward biopotential stimulation, recording, and telemetry. He presently works as a Senior Analog IC Designer with a focus on analog ASIC design for wireless neuro and biomedical engineering for Cirtec Medical, Inc., Brooklyn Park, MN, USA.

Kirk Gronda specializes in the orthopedic, spinal, interventional, and neuromodulation medical device sectors. With experience in technology strategy, product development, and market intelligence, he plays a key role in shaping technology roadmaps and driving business development initiatives. His experience spans cross-functional leadership, competitive intelligence, and value-driven commercialization strategies, supporting the development of next-generation intelligent implants and neuromodulation devices.